Commercialization of radiation polymerizable coatings, inks and films requires the reactants be cured quickly and completely.
Typically, such photopolymerizable compositions contain a photosensitive monomeric and/or polymeric material along with a photoinitiator and adjuvant materials which provide desired properties for the end product coating or film.
If the cure of the reactants is not complete, the residues of unreacted starting materials migrate out of the coating following cure. This is disadvantageous since such residues cause contamination of the environment or otherwise render the coating unsuitable for use in connection with products having direct food contact. As a result, efforts have been made to develop UV curable coating compositions that cure as quickly and completely as possible.
It has been found that photopolymerization of a charge transfer complex composition may be achieved with ultra-violet light and without the need for addition of a photoinitiating compound. More particularly, U.S. Pat. No. 5,446,073 to Jonsson et al. describes charge transfer complexes obtained from at least one unsaturated compound that has an electron donor group and an electron withdrawing group. In a preferred embodiment, the specific electron donating material is a vinyl ether and the electron withdrawing compound is a maleamide. These compositions cure in the absence of a photoinitiator upon subjecting the composition to ultra-violet light having a defined wavelength.
More recently, ultra-violet curing of vinyl ether maleate systems have been developed which incorporate free radical photoinitiators. For example, EP 0 322 808 B1 to Friedlander et al. discloses a film or liquid radiation curable composition comprising an ethylenically unsaturated polyester component and ethylenically unsaturated polyester oligomer component in a nonpolymerized vinyl ether component together with the free radical photoinitiator.
A similar vinyl ether maleate system is disclosed in PCT/NL97/00017 to Jansen whereby the radiation curable binder comprises an unsaturated compound containing at least one maleate and an unsaturated compound comprising at least one vinyl ether together with the free radical photoinitiator.
While the above systems provide a modest increase in cure time for the reactants, each falls far short of the cure rates required for purposes of commercialization. Further, a substantial quantity of unreacted vinyl ether and polyester remain following irradiation notwithstanding the use of a free radical photoinitiator. It has been observed that as much as 20% to 40% of unreacted vinyl ether or unsaturated polyester remains un-crosslinked in the resultant polymeric films.
It is known to employ a cationic catalyst or photoinitiator for copolymerization of vinyl ethers and unset polyesters. A small amount of a Lewis Acid catalyst such as Sn Co.sub.4 will readily polymerize vinyl monomers such as styrene butadiene, vinyl alkyl ethers or the like. For example, U.S. Pat. No. 4,423,136 to Crivello provides a cationic photoinitiator in connection with ultra-violet cure of unsaturated polyesters including vinyl ethers.
In view of the above, a need has existed in the art for an improved photopolymerization process and composition having increased cure rates and conversion of the reactants with a reduction in the release of volatiles.